As a production process for an optically active 2-halogeno carboxylic acid, the following production process has been disclosed, for example, for an optically active 2-bromoisovaleric acid.
Organic Process Research & Development (1998), 2, 238-244, discloses a process of halogenating an optically active valine in water in the presence of a nitrous acid with steric configuration being retained, then extracting the formed optically active 2-bromoisovaleric acid with methyl t-butyl ether, then adding diisopropyl amine thereby forming and, at the same time, crystallizing a diisopropyl amine salt of an optically active 2-bromoisovaleric acid (optical purity of obtained crystals: about 98 to 98.5% ee.)
However, as a result of the studies made by the present inventors, it has been found that the method described above involves the following various problems which give an effect on the quality.    (1) The optical purity of the optically active 2-bromoisovaleric acid obtained is low due to racemization during reaction. Further, it is not always easy to improve the chemical purity and optical purity by purification of the amine salt.    (2) The optically active 2-bromoisovaleric acid tends to be decomposed in successive steps of reaction to crystallization to produce 2-hydroxy carboxylic acid and bromine ingredients as by-products.    (3) The 2-hydroxyisovaleric acid produced as a by-product is difficult to be removed by extraction and tended to accompany as far as final crystallization step.    (4) The bromine ingredients are difficult to be removed in extraction to crystallization and tended to intrude into products.
The foregoing problems give a large burden on the final crystallization step. It is difficult to obtain an optically active 2-halogenocarboxylic acid at high quality in the method described above, and a laborious purification step such as column chromatography or re-crystallization is additionally required for improving the quality.